1. Field of the Invention
This invention relates to an engine output controlling system for a vehicle wherein driving torque of an engine is reduced rapidly in response to an amount of slip of a driving wheel upon acceleration or the like of the vehicle in order to assure safe running of the vehicle.
2. Description of the Prior Art
When the condition of a road on which a vehicle is driving varies suddenly or when a vehicle is driving on a slippery road having a low coefficient of friction such as, for example, a snowy road or a frozen road, it frequently occurs that a driving wheel of the vehicle rotates slipping or in vain.
In such an instance, it is very difficult even for a skilled driver to adjust the amount of operation of an accelerator pedal to control the output power of an engine skillfully so that the driving wheel may not rotate slipping or in vain.
From such circumstances, several improved engine output controlling systems have been proposed, and in an exemplary one of such systems, a slipping condition of a driving wheel is detected and, if the slip of the driving wheel exceeds a predetermined level, then the output power of the engine is reduced compulsorily independently of an amount of operation of the accelerator pedal by the driver. According to such engine output controlling systems, the driver can select, when it is considered necessary, one of driving of the vehicle in which the engine output controlling system is used and normal driving in which the output power of the engine is controlled in response to an amount of operation of the accelerator pedal.
In one of such vehicle engine output controlling systems, a rotational speed of a driving wheel and another rotational speed of a driven wheel are detected and, considering a difference between the rotational speeds as an amount of slip of the driving wheel, the driving torque of the engine is controlled in response to such slip amount. In short, the conventional engine output controlling system presumes a condition of the road in accordance with a variation of the rotating condition of the driven wheel, sets a driving torque which should serve as a reference of the engine corresponding to the condition of the road corrects the thus set driving torque serving as a reference torque in accordance with a difference between rotational speeds of the driving wheel and the driven wheel, and controls torque controlling means so that the driving torque of the engine may be equal to the thus corrected driving torque. The conventional engine output controlling system further sets an aimed driving torque of the engine in accordance with the slip amount detected as above and controls the opening of a throttle valve, the ignition timing and so forth so that the driving torque of the engine may be equal to the aimed driving torque.
With such conventional engine output controlling system for a vehicle as described above, when the increasing tendency of the slip amount of the driving wheel is very high, a retarding operation of an ignition timing which presents a smaller delay in control than opening control of a throttle valve or the like is performed so that the driving torque of the engine may quickly become equal to the aimed driving torque.
In such conventional engine output controlling system for a vehicle, a condition of a road is presumed in accordance with a variation of the rotating condition of the driven wheel as described above. However, the actual road may present any one of very different conditions, and accordingly, it is very difficult as a matter of fact to presume a coefficient of friction of a tire with respect to a road.
Therefore, actually a large number of correction factors must be adopted to specify control requirements, and accordingly, there is the possibility that such disadvantages may take place that the program for the control is complicated, which will cause some delay in control or the cost for a calculating unit is raised.
Meanwhile, in a conventional engine output controlling system wherein an aimed driving torque of an engine is set in accordance with an amount of slip of a driving wheel and a driving condition of the vehicle and then the opening of a throttle valve, the ignition timing or the like is controlled so that the driving torque of the engine may be equal to the aimed driving torque, even when the increasing tendency of the slip amount of the driving wheel is very high, such control as to lower the aimed driving torque to a level for idling or so is not performed fundamentally.
As a result, when a slip of the driving wheel takes place suddenly on ice or the like, it is difficult to suppress the slip rapidly by an ignition timing retarding operation or the like, and accordingly, a considerable delay cannot be avoided at any cost.
In case the driving condition of the vehicle when the increasing tendency of the slip amount of the driving wheel is very high, for example, on ice or the like, is taken into consideration, even if the driving torque of the engine is lowered to a level for idling or so, it is desirable at any rate to suppress the slip rapidly to assure the controllability of the vehicle.
By the way, when a vehicle is driving but not at a very low speed, the driving wheel slips more or less with respect to a road. However, it is well known also by experience that, if a driving torque higher than a frictional force between the road and the driving wheel, then the slip amount of the driving wheel is increased suddenly to make the driving of the vehicle difficult.